The multistage evolution of squamous skin tumors induced by chemical or viral carcinogens on mice from different genetic backgrounds has indicated that susceptibility determinants are multigenic, stage specific, dependent on the carcinogenesis protocol, and in the case of initiating events, intrinsic properties of keratinocytes. Malignant conversion of neoplastic keratinocytes derived from FVB/N, inbred SENCARA/Pt, BALB/c, or C57BL/6 mouse strains differ substantially in the frequency of progression from papilloma to carcinoma. In vitro, FVB/N keratinocytes were 10-fold more sensitive to chemically induced malignant conversion than keratinocytes from other strains, and formed squamous cell carcinomas at a higher frequency after immortalization with HPV-16 E6/E7 genes, consistent with the known sensitivity of this strain to premalignant progression in vivo. Induction of skin tumors on F1 offspring of crosses from SENCARA/Pt and FVB/N mice indicate that predisposition to premalignant progression is not a dominant characteristic. This predisposition is an intrinsic property of the target keratinocytes and as such should be amenable to further study in isolated cells.Changes in AP-1 transcriptional activity have been strongly associated with malignant conversion in skin carcinogenesis. Transgenic mice that conditionally express an inhibitor (A-FOS) of all AP-1 activity in the skin using the bovine keratin 5 (K5) promoter for targeting and a tetracycline transactivator for regulation were generated. When A-FOS is expressed during chemical-induced skin carcinogenesis, mice develop sebaceous adenomas containing a signature mutation in the H-ras proto-oncogene. Carcinogenesis in the absence of A-FOS produces squamous papillomas with the same mutation. When A-FOS is expressed following tumor formation, squamous papillomas transdifferentiate into sebaceous adenomas. Suppressing A-FOS after sebaceous adenoma formation causes transdifferentiation into squamous papillomas. Whenever A-FOS is expressed, malignant progression is inhibited. Gene expression data from cultured control and transgenic keratinocytes and protein localization in tumors indicate that AP-1 regulates the balance between the Wnt and Hedgehog signaling pathways that are known to determine squamous and sebaceous lineages. Thus AP-1 activity is essential for determining both the differentiated state of pluripotential tumor cells and progression to malignancy. CLIC4 is a cytoplasmic and mitochondrial inner membrane protein that belongs to the CLIC family of intracellular organelle chloride channels. In keratinocytes and other cell types, CLIC4 is upregulated by p53, c-myc, TNFalpha, DNA damage and differentiation or senescence coinciding with the onset of growth arrest or apoptosis. Overexpression of CLIC4 in keratinocytes induces apoptosis through a mitochondrial death pathway. These same stress-related stimuli cause cytoplasmic CLIC4 to translocate to the nucleus utilizing an intrinsic nuclear localization sequence and the Ran/importin transport complex. Direct targeting of CLIC4 to the nucleus accelerates apoptosis and can bypass the mitochondrial pathway to induce apoptosis in Apaf null cells. CLIC4 is a direct response gene for p53 and c-myc through functional binding sites on the CLIC4 promoter. Upregulation of CLIC4 is essential for apoptosis induced by p53 or c-myc. CLIC4 mRNA expression is reduced in breast, ovarian and renal cancers, but human tumor tissue arrays and the NCI 60 cell lines show variability in CLIC4 protein expression, indicating that CLIC4 may be post-translationally regulated in tumors. CLIC4 is predominantly nuclear in normal and benign lesions and cytoplasmic in cancers. Cultured human and mouse cancer cells are deficient nuclear translocation of CLIC4 in response to multiple stress stimuli and ras transformation of normal keratinocytes delays nuclear translocation of CLIC4.